It is known that polyurethanes may be produced by reacting polyisocyanates with compounds containing from 2 to 6 OH-groups and having a molecular weight of from 62 to about 10,000. Examples of these polyhydroxy compounds are difunctional and more highly functional alcohols, such as ethylene glycol, diethylene glycol, hexane diol, glycerol and trimethylol propane, and also relatively high molecular weight polyethers, polythioethers, polyesters and polyacetals. The relatively high molecular weight polyhydroxy compounds are produced in known manner from low molecular weight units.
In these polyhydroxy compounds, the various OH-functions are generally equivalent in regard to the reactivity thereof and the distance thereof from any branching center present. Exceptions are low molecular weight alcohols containing primary and secondary hydroxy groups, such as glycerol. Although, in the case of relatively high molecular weight polyethers and polyesters, both primary and secondary OH-groups are also present in many cases, the distribution thereof is statistical so that it is not possible to synthesize polymers having a defined structure on account of this difference in reactivity. The chain length distributions in branched polyethers and polyesters are also statistical.
It is also known that the above-mentioned polyhydroxy compounds may be extended by a sub-molar quantity of a polyisocyanate to form OH-prepolymers. Although branching occurs in the case of trifunctional isocyanates, the reactivity of the OH-groups and the chain length distribution are again statistical.
In addition, the separate production of OH-prepolymers for the subsequent production of polyurethanes is generally not advisable because the same polyurethane structures are formed where production is carried out by the one-shot process or via NCO-prepolymers.
The production of polyurethanes and, in particular, the production of sterically cross-linked polyurethanes requires polyhydroxy compounds which contain OH-groups of different reactivity and chain branches of different length. It would be advantageous, for example, if trifunctional polyhydroxy compounds were available which contained two OH-groups of high reactivity at the ends of the main chain and an OH-function of reduced reactivity in as short a side chain as possible. Such a structure could be expected to give a polymer having particularly favorable mechanical properties. It is also desirable to be able to use polyhydroxy compounds which give polyurethanes having improved flame resistance. Furthermore, there is a need for OH-prepolymers which, in the event of hydrolytic degradation, do not form toxic aromatic diamines. The present invention provides a solution to these problems.